LINEARITY OF FATIGUE CRACK GROWTH RATES IN THE NEAR-THRESHOLD REGIME

Based upon comprehensive analysis of our experimental data and of literature data, we postulate that many near-threshold fatigue crack growth data are represented very well by a modified Paris law in terms of a composite driving force of the Walker form, that depends on both cyclic stress intensity Delta K and maximum stress intensity K-MAX-In the near-threshold regime, the leading terms in the crack growth rate are linear in terms of this composite driving force. Deviations from linear behavior in the near-threshold regime are associated with crack growth rate dependent environmental influences. This alternative method of threshold determination explicitly depends on both Delta K and K-MAX, in contrast to a single threshold defined in terms of Delta K alone. We discuss the implications of this near-threshold behavior in the context of the new Unigrow model of Noroozi, Glinka & Lambert and the Unified Approach of Sadananda & Vasudevan.